Packaging containers for liquid foods, such as milk and juice, are produced with the aid of modern, rational filling machines which, either from a web or from prefabricated sheets of a packaging laminate, form, fill and seal the packages.
Examples of common packaging containers for milk, juice and other liquid foods are Tetra Brik, Tetra Rex and Tetra Top (all registered trademarks).
From, for example, a web of the packaging material, parallelepipedic packaging containers of the Tetra Brik type are produced in that the web is first reformed into a tube by both opposing longitudinal edges of the web being folded towards and, by thermosealing, permanently united to one another in a liquid-tight overlap joint. The tube is filled with the pertinent contents, for example milk, at the same time as the tube is divided into continuous, filled cushion-shaped packaging units by repeated flat-pressing operations and sealings of the tube in transverse sealing zones across the longitudinal direction of the tube below the level of the contents in the tube. The packaging units are separated from one another by incisions in the transverse sealing zones and are given the desired geometric configuration, usually parallelepipedic, by inward folding and fixing of the double-walled triangular corner flaps against each respective adjacent planar packaging wall or panel. The finished packaging containers are thereafter discharged from the filling machine for further transport and handling.
In tetrahedral packaging containers the transverse sealing of the tube takes place substantially at right angles to the longitudinal direction of the tube and alternating in spaced apart relationship from each other in two planes at right angles to each other.
Conventional packaging containers of the above-described types are produced from a laminated packaging material comprising a rigid, but foldable, core or bulk layer of paper or paperboard and outer, liquid-tight coatings of thermoplastic, preferably polyethylene. For particularly foods sensitive to oxygen gas, such as for example, juice, wine and cooking oils, the packaging material moreover includes at least one additional layer by means of which the requisite tightness properties against oxygen are ensured.
In both the above types of package containers the longitudinal overlap joint which is formed on the tube entails that the incision surface of the inner longitudinal edge of the packaging material will be exposed to the contents of the package, which implies that the contents may readily be absorbed into the packaging material if this includes a fibrous material such as, for example, paper or paperboard. Another problem is that the contents of the package may come into contact with any possible metal layer in the packaging material.
To avoid direct contact between the incision edge and the contents of the package a sealing strip is applied and fixed in the filling machine along one longitudinal edge of the packaging material web, so that it has a free strip edge projecting from the longitudinal edge. The planar packaging material web provided with the strip is then reformed into a tube in that, as was described above, both of the longitudinal edges of the web are folded towards and permanently united to one another in an overlap seal or joint. During the tube forming operation in the filling machine, the projecting free strip edge is folded into planar abutment against the overlapping inside of the second longitudinal web edge and is fixedly sealed thereto by thermosealing in such a manner that the incision edge of the first longitudinal web edge facing towards the interior of the tube is completely covered and protected against liquid penetration (edge wicking).
In order to give protection against liquid penetration, the sealing strip must thus be thermosealable to counterfacing sealing surfaces of the packaging material and an additional requirement is that the thermosealing must be capable of being carried out in an efficient and expedient manner even at the extremely high production output speeds at which today's modern filling machines operate. The sealing strip is normally laminated and may be made of many different materials.
One prior art sealing strip that is employed in a commercial packaging container of a packaging material comprising a paper- or paperboard layer and outer liquid-tight coatings of polyethylene, preferably low density polyethylene (LDPE) has a base layer of polyethylene terephthalate (PET) and outer, thermosealable plastic coatings of polyethylene, for example low density polyethylene (LDPE).
Another prior art sealing strip for a commercial packaging material of the above-described type has a base layer of polyethylene terephthalate (PET) and outer, thermosealable plastic coatings of metallocene polyethylene (mLLDPE) which, in comparison with low density polyethylene (LDPE) displays an advantageously wider sealing window than LDPE with a lower temperature limit that lies below the corresponding limit for LDPE.
A further prior art sealing strip comprises a base layer of a polymer possessing gas barrier properties which, on its one side, has a first sealing layer of polyethylene and, on its other side, has a second sealing layer of polyethylene. The first and second sealing layers may display a two layer structure consisting of an outer layer of a mixture of metallocene polyethylene (mLLDPE), low density polyethylene (LDPE) and/or a density-increasing polyethylene component and an inner layer of a mixture of low density polyethylene (LDPE) and/or a density-increasing polyethylene component.
In the production of the sealing strip it sometimes happens that the strip is broken. If that happens one often splices the ends of the strip in order to not having to discard larger lengths of the sealing strip. It is also possible to splice several, not broken, sealing strips, to increase the total length of the sealing strip formed. Today the normal method of splicing is to cut the free ends of the strips transversely as seen in plan view, i.e. 90° in relation to the longitudinal direction of the sealing strip. The free ends of the sealing strips are then placed on top of each other with an overlap of about 7-10 mm. The ends are then heated (welded), to fuse the ends together. One problem that may occur, depending on the skill of the operator, is that the overlap becomes too long and that the heating does not cover the total length of the overlap at the spliced area. If the heating does not cover the total overlap, the ends of the strip parts to be spliced may not be fused to the sealing strip, with the potential risk that the splice will disintegrate. In the overlapping area the sealing strip will have double thickness, giving an increased stiffness. The increased stiffness may have negative influence in the forming of packaging containers, in that the sealing strip does not bend smoothly. There may also be problems if the overlapping area is placed at a crease line of the packaging material web.